Since the carbide is being gradually converted into damp slaked
lime, it increases in weight to an indeterminate extent as the generator
in exhausted; but since, on the other hand, some lime may be washed out
of the basket each time it is submerged, and some of the smaller
fragments of carbide may fall through the perforations, the basket tends
to decrease in weight as the generator is exhausted. Thus it happens in
A^2 that the combined weight of bell plus basket plus contents is wholly
indefinite, and the pressure in the service becomes so irregular that a
separate governor must be added to the installation before the burners
can be expected to behave properly. In the third place, the water in the
tank serves both for generation and for decomposition, and this involves
the employment of some arrangement to keep its level fairly constant lest
the bell should become unsealed, while protection from frost by saline or
liquid additions is impossible. A^2 is known popularly as a "dipping"
generator, and it will be seen to be defective mechanically and bad
chemically. In both A^1 and A^2 the bell is constructed of thin sheet-
metal, and it is cylindrical in shape; the mass of metal in it is
therefore negligible in comparison with the mass of water in the tank,
and so the level of the liquid is sensibly the same whether the bell be
high or low.
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